Light emitting devices are used in many different applications, including projection display systems, backlights for liquid crystal displays and the like. Projection systems typically use one or more white light sources, such as high pressure mercury lamps. The white light beam is usually split into three primary colors, red, green and blue, and is directed to respective image forming spatial light modulators to produce an image for each primary color. The resulting primary-color image beams are combined and projected onto a projection screen for viewing.
More recently, light emitting diodes (LEDs) have been considered as an alternative to white light sources. LEDs have the potential to provide the brightness and operational lifetime that would compete with conventional light sources. Current LEDs, however, especially green, yellow and amber emitting LEDs, are relatively inefficient. Furthermore, conventional light sources typically have spectral characteristics that vary as a function of output light intensity. Often, complicated and expensive means are employed to stabilize the spectral characteristics, such as the color, of the emitted light.